The present invention relates to an improvement in an electric motor actuator in which an output shaft of a reduction gear unit is coupled to such a machine element as a robot arm.
FIG. 1 shows, in a model form, a conventionally known actuator which is a combination of an electric motor and a reduction gear unit. In the drawing, a reduction gear unit 2 is driven at its primary side by an output shaft of an electric motor 1 and is coupled at its secondary side to such a machine element 3 as a robot arm so that the machine element can be eventually set or shifted at or to a desired position. And the control of the actuator is effected by adjusting the rotational angle of the shaft of the motor. More specifically, encoders E.sub.1 and E.sub.2 are mounted respectively on the rotating shaft of the motor and on the output shaft of the reduction unit. A method of controlling the actuator by counting the number of pulses of the each encoder is excellent in the operational accuracy of the actuator, since it can advantageously correct the backlash and torsion caused by the reduction unit.
According to this method, pulse signals generated from the two encoders during rotation of the shafts of the motor and reduction unit are processed to find a difference in rotational angle between the motor shaft and the reduction unit shaft and thereby to know the degree or magnitude of the backlash and torsion, whereby the state of the actuator is recognized.
However, this method has a defect that the whole control circuit becomes more complex than that in a method for controlling the motor alone, since the pulse processing circuit for detecting the rotational angle difference becomes complex.